Multicast trunking in a network device

ABSTRACT

A network device for uniform distribution of non-unicast traffic, such as layer  2  broadcast, layer  2  multicast, unknown unicast and layer  3  multicast on a truch group. The network device includes at least one trunk group including a plurality of physical ports. The network device is connected to at least one other network device by a number of the plurality of physical ports. The network device also includes a table with a plurality of entries, wherein each entry is associated with the number of the plurality of physical ports on the network device. Each entry specifies if a packet should be transmitted on one of the plurality of physical ports. The network device further includes hashing means for hashing a predefined number of bits from predefined fields in the packet to select one entry in the table. The selected entry is used to identify a destination port. The network device also includes transmitting means for transmitting the packet to the destination port.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority of U.S. Provisional Patent ApplicationSer. No. 60/631,578, filed on Nov. 30, 2004 and U.S. Provisional PatentApplication Ser. No. 60/686,425, filed on Jun. 2, 2005. The subjectmatter of this earlier filed application is hereby incorporated byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a network device in a data network andmore particularly to a system and method of logically linking multipleports into a signal port of the network device and transmittingmulticast packets through the logical port.

2. Description of the Related Art

A packet switched network may include one or more network devices, suchas a Ethernet switching chip, each of which includes several modulesthat are used to process information that is transmitted through thedevice. Specifically, the device includes an ingress module, a MemoryManagement Unit (MMU) and an egress module. The ingress module includesswitching functionality for determining to which destination port apacket should be directed. The MMU is used for storing packetinformation and performing resource checks. The egress module is usedfor performing packet modification and for transmitting the packet to atleast one appropriate destination port. One of the ports on the devicemay be a CPU port that enables the device to send and receiveinformation to and from external switching/routing control entities orCPUs.

A current network device may support physical ports and logical/trunkports, wherein the trunk ports are a set of physical external ports thatact as a single link layer port. Ingress and destination ports on thenetwork device may be physical external ports or trunk ports. Bylogically combining multiple physical ports into a trunk port, thenetwork may provide greater bandwidth for connecting multiple devices.Furthermore, if one port in the trunk fails, information may still besent between connected devices through other active ports of the trunk.As such, trunk ports also enable the network to provide greaterredundancy between connected network devices.

In order to transmit information from one network device to another, thesending device has to determine if the packet is being transmitted to atrunk destination port. If the destination port is a trunk port, thesending network device must dynamically select a physical external portin the trunk on which to transmit the packet. The dynamic selection mustaccount for load sharing between ports in a trunk so that outgoingpackets are distributed across the trunk.

Typically, each packet entering a network device may be one of a unicastpacket, a broadcast packet, a muliticast packet, or an unknown unicastpacket. The unicast packet is one that is to be transmitted to aspecific destination address that can be determined by an ingressnetwork device. The broadcast packet is typically sent to all ports bythe ingress network device and the multicast packet is sent to multipleidentifiable ports by the ingress network device. To multicast orbroadcast a packets specific bits in the packet are set prior totransmission of the packet to the ingress network device. An unknownunicast packet is a unicast packet in which the ingress network devicecannot determine the port associated with the destination address. Sothe ingress network device broadcasts the packet which is ignored by allports except the intended but previously unknown destination port. Whenthe previously unknown destination port sends a response message to theingress network device, all network devices “learn” the associateddestination address. Thereafter, any unicast packet sent to thepreviously unknown port is transmitted as a traditional unicast packet.

When a broadcast/multicast packet is to be sent on a trunk group thatincludes multiple physical ports, the ingress device must send thepacket to every device in the network with sending duplicate copies ofthe packet on a given trunk. Current network devices include two typesof multicast methods: a layer 2 multicast/broadcast/unknown unicast anda layer 3 multicast. As such, network devices include three masks: onefor layer 3, layer 2 and broadcast. The network devices use these masksto select a destination port on which to send the packet. However, thesemasks are limiting and do not provide for adequate distribution of thetraffic on a given trunk group

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this specification, illustrate embodiments of the invention thattogether with the description serve to explain the principles of theinvention, wherein:

FIG. 1 illustrates a network device in which an embodiment of thepresent invention may be implemented;

FIG. 2 illustrates a centralized ingress pipeline architecture,according to one embodiment of the present invention;

FIG. 3 illustrates an embodiment of the network in which multiplenetwork devices are connected by trunks; and

FIGS. 4 a-4 c illustrate a trunk group table that is used in anembodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Reference will now be made to the preferred embodiments of the presentinvention, examples of which are illustrated in the accompanyingdrawings.

FIG. 1 illustrates a network device, such as a switching chip, in whichan embodiment the present invention may be implemented. Device 100includes an ingress module 102, a MMU 104, and an egress module 106.Ingress module 102 is used for performing switching functionality on anincoming packet. MMU 104 is used for storing packets and performingresource checks on each packet. Egress module 106 is used for performingpacket modification and transmitting the packet to an appropriatedestination port. Each of ingress module 102, MMU 104 and Egress module106 includes multiple cycles for processing instructions generated bythat module. Device 100 implements a pipelined approach to processincoming packets. The device 100 has the ability of the pipeline toprocess, according to one embodiment, one packet every clock cycle.According to one embodiment of the invention, the device 100 includes a133.33 MHz core clock. This means that the device 100 architecture iscapable of processing 133.33M packets/sec.

Device 100 may also include one or more internal fabric high speedports, for example a HiGig™, high speed port 108 a-108 x, one or moreexternal Ethernet ports 109 a-109 x, and a CPU port 110. High speedports 108 a-108 x are used to interconnect various network devices in asystem and thus form an internal switching fabric for transportingpackets between external source ports and one or more externaldestination ports. As such, high speed ports 108 a-108 x are notexternally visible outside of a system that includes multipleinterconnected network devices. CPU port 110 is used to send and receivepackets to and from external switching/routing control entities or CPUs.According to an embodiment of the invention, CPU port 110 may beconsidered as one of external Ethernet ports 109 a-109 x. Device 100interfaces with external/off-chip CPUs through a CPU processing module111, such as a CMIC, which interfaces with a PCI bus that connectsdevice 100 to an external CPU.

Network traffic enters and exits device 100 through external Ethernetports 109 a-109 x. Specifically, traffic in device 100 is routed from anexternal Ethernet source port to one or more unique destination Ethernetports 109 a-109 x. In one embodiment of the invention, device 100supports physical Ethernet ports and logical (trunk) ports. A physicalEthernet port is a physical port on device 100 that is globallyidentified by a global port identifier. In an embodiment, the globalport identifier includes a module identifier and a local port numberthat uniquely identifies device 100 and a specific physical port. Thetrunk ports are a set of physical external Ethernet ports that act as asingle link layer port. Each trunk port is assigned a global a trunkgroup identifier (TGID). According to an embodiment, device 100 cansupport up to 128 trunk ports, with up to 8 members per trunk port, andup to 29 external physical ports. Destination ports 109 a-109 x ondevice 100 may be physical external Ethernet ports or trunk ports. If adestination port is a trunk port, device 100 dynamically selects aphysical external Ethernet port in the trunk by using a hash to select amember port. As explained in more detail below, the dynamic selectionenables device 100 to allow for dynamic load sharing between ports in atrunk.

Once a packet enters device 100 on a source port 109 a-109 x, the packetis transmitted to ingress module 102 for processing. Packets may enterdevice 100 from a XBOD or a GBOD. In an embodiment, the XBOD is a blockthat has one 10GE/12G MAC and supports packets from high speed ports 108a-108 x. The GBOD is a block that has 12 10/100/1G MAC and supportspackets from ports 109 a-109 x.

FIG. 2 illustrates a centralized ingress pipeline architecture 200 ofingress module 102. Ingress pipeline 200 processes incoming packets,primarily determines an egress bitmap and, in some cases, figures outwhich parts of the packet may be modified. Ingress pipeline 200 includesa data holding register 202, a module header holding register 204, anarbiter 206, a configuration stage 208, a parser stage 210, a discardstage 212 and a switch stage 213. Ingress pipeline 200 receives datafrom the XBOD, GBOD or CPU processing module 111 and stores cell data indata holding register 202. Arbiter 206 is responsible for schedulingrequests from the GBOD, the XBOD and CPU. Configuration stage 208 isused for setting up a table with all major port-specific fields that arerequired for switching. Parser stage 210 parses the incoming packet anda high speed module header, if present, handles tunnelled packetsthrough Layer 3 (L3) tunnel table lookups, generates user definedfields, verifies Internet Protocol version 4 (IPv4) checksum on outerIPv4 header, performs address checks and prepares relevant fields fordownstream lookup processing. Discard stage 212 looks for various earlydiscard conditions and either drops the packet and/or prevents it frombeing sent through pipeline 200. Switching stage 213 performs all switchprocessing in ingress pipeline 200, including address resolution.

According to one embodiment of the invention, the ingress pipelineincludes one 1024-bit cell data holding register 202 and one 96-bitmodule header register 204 for each XBOD or GBOD. Data holding register202 accumulates the incoming data into one contiguous 128-byte cellprior to arbitration and the module header register 204 stores anincoming 96-bit module header for use later in ingress pipeline 200.Specifically, holding register 202 stores incoming status information.

Ingress pipeline 200 schedules requests from the XBOD and GBOD every sixclock cycles and sends a signal to each XBOD and GBOD to indicate whenthe requests from the XBOD and GBOD will be scheduled. CPU processingmodule 111 transfers one cell at a time to ingress module 102 and waitsfor an indication that ingress module 102 has used the cell beforesending subsequent cells. Ingress pipeline 200 multiplexes signals fromeach of XBOD, GBOD and CPU processing based on which source is grantedaccess to ingress pipeline 200 by arbiter 206. Upon receiving signalsfrom the XBOD or GBOD, a source port is calculated by register buffer202, the XBOD or GBOD connection is mapped to a particular physical portnumber on device 100 and register 202 passes information relating to ascheduled cell to arbiter 206.

When arbiter 206 receives information from register buffer 202, arbiter206 may issue at least one of a packet operation code, an instructionoperation code or a FP refresh code, depending on resource conflicts.According to one embodiment, the arbiter 206 includes a main arbiter 207and auxiliary arbiter 209. The main arbiter 207 is a time-divisionmultiplex (TDM) based arbiter that is responsible for schedulingrequests from the GBOD and the XBOD, wherein requests from main arbiter207 are given the highest priority. The auxiliary arbiter 209 schedulesall non XBOD/GBOD requests, including CPU packet access requests, CPUmemory/register read/write requests, learn operations, age operations,CPU table insert/delete requests, refresh requests and rate-limitcounter refresh request. Auxiliary arbiter's 209 requests are scheduledbased on available slots from main arbiter 207.

When the main arbiter 207 grants an XBOD or GBOD a slot, the cell datais pulled out of register 202 and sent, along with other informationfrom register 202, down ingress pipeline 200. After scheduling theXBOD/GBOD cell, main arbiter 207 forwards certain status bits toauxiliary arbiter 209.

The auxiliary arbiter 209 is also responsible for performing allresource checks, in a specific cycle, to ensure that any operations thatare issued simultaneously do not access the same resources. As such,auxiliary arbiter 209 is capable of scheduling a maximum of oneinstruction operation code or packet operation code per request cycle.According to one embodiment, auxiliary arbiter 209 implements resourcecheck processing and a strict priority arbitration scheme. The resourcecheck processing looks at all possible pending requests to determinewhich requests can be sent based on the resources that they use. Thestrict priority arbitration scheme implemented in an embodiment of theinvention requires that CPU access request are given the highestpriority, CPU packet transfer requests are given the second highestpriority, rate refresh request are given the third highest priority, CPUmemory reset operations are given the fourth highest priority and Learnand age operations are given the fifth highest priority by auxiliaryarbiter 209. Upon processing the cell data, auxiliary arbiter 209transmits packet signals to configuration stage 208.

Configuration stage 208 includes a port table for holding all major portspecific fields that are required for switching, wherein one entry isassociated with each port. The configuration stage 208 also includesseveral registers. When the configuration stage 208 obtains informationfrom arbiter 206, the configuration stage 208 sets up the inputs for theport table during a first cycle and multiplexes outputs for other portspecific registers during a second cycle. At the end of the secondcycle, configuration stage 208 sends output to parser stage 210.

Parser stage 210 manages an ingress pipeline buffer which holds the128-byte cell as lookup requests traverse pipeline 200. When the lookuprequest reaches the end of pipeline 200, the data is pulled from theingress pipeline buffer and sent to MMU 104. If the packet is receivedon a high speed port, a 96-bit module header accompanying the packet isparsed by parser stage 210. After all fields have been parsed, parserstage 210 writes the incoming cell data to the ingress pipeline bufferand passes a write pointer down the pipeline. Since the packet data iswritten to the ingress pipeline buffer, the packet data need not betransmitted further and the parsed module header information may bedropped. Discard stage 212 then looks for various early discardconditions and, if one or more of these conditions are present, discardstage drops the packet and/or prevents it from being sent through thechip.

Switching stage 213 performs address resolution processing and otherswitching on incoming packets. According to an embodiment of theinvention, switching stage 213 includes a first switch stage 214 and asecond switch stage 216. First switch stage 214 resolves any dropconditions, performs BPDU processing, checks for layer 2 source stationmovement and resolves most of the destination processing for layer 2 andlayer 3 unicast packets, layer 3 multicast packets and IP multicastpackets. The first switch stage 214 also performs protocol packetcontrol switching by optionally copying different types of protocolpackets to the CPU or dropping them. The first switch stage 214 furtherperforms all source address checks and determines if the layer 2 entryneeds to get learned or re-learned for station movement cases. The firstswitch stage 214 further performs destination calls to determine how toswitch packet based on a destination switching information.Specifically, the first switch stage 214 figures out the destinationport for unicast packets or port bitmap of multicast packets, calculatesa new priority, optionally traps packets to the CPU and drops packetsfor various error conditions. The first switch stage 214 further handleshigh speed switch processing separate from switch processing from port109 a-109 i and switches the incoming high speed packet based on thestage header operation code.

The second switch stage 216 then performs Field Processor (FP) actionresolution, source port removal, trunk resolution, high speed trunking,port blocking, CPU priority processing, end-to-end Head of Line (HOL)resource check, resource check, mirroring and maximum transfer length(MTU) checks for verifying that the size of incoming/outgoing packets isbelow a maximum transfer length. The second switch stage 216 takes firstswitch stage 216 switching decision, any layer routing information andFP redirection to produce a final destination for switching. The secondswitch stage 216 also removes the source port from the destination portbitmap and performs trunk resolution processing for resolving thetrunking for the destination port for unicast packets, the ingressmirror-to-port and the egress mirror-to-port. The second switch stage216 also performs high speed trunking by checking if the source port ispart of a high speed trunk group and, if it is, removing all ports ofthe source high speed trunk group. The second switch stage 216 furtherperforms port blocking by performing masking for a variety of reasons,including meshing and egress masking.

FIG. 3 illustrates an embodiment of a network in which multiple networkdevices, as described above, are connected by trunks. According to FIG.3, network 300 includes devices 302-308 which are connected by trunks310-316. Device 302 includes ports 1 and 2 in trunk group 310, device304 includes ports 4 and 6 in trunk group 310 and device 306 includesports 10 and 11 in trunk group 310. Each of network devices 302-308 mayreceive unicast or multicast packets that must be transmitted to anappropriate destination port.

As noted above, an embodiment of device 100 may support up to 128 trunkports with up to 8 members per trunk port. When the incoming packet is amulticast, broadcast or unknown unicast packet, the receiving deviceuses an associated trunk block table 400, as illustrated in FIGS. 4 a-4c. There are two types of multicast implemented in an embodiment of theinvention: multicast/broadcast/unknown unicast for layer 2, and layer 3multicast.

In an embodiment of the invention, trunk block table 400 is a 16 entrytable that includes a block mask for specifying whether or not a packetshould be sent on a given trunk port. Therefore, trunk block table 400enables a sending device to select only one port, in a given trunkgroup, for forwarding a packet. One embodiment of the invention teachesthat each of devices 302-308 has it own trunk block table 400. Forexample, trunk group table 4 a is associated with device 302, trunkgroup table 4 b is associated with device 304 and trunk group table 4 cis associated with device 306. According to the illustrations in FIG. 3,if a packet is to be sent out on port 11 of trunk group 310, an entry402 a is set in trunk block table 400, as shown in FIG. 4 a, associatedwith device 302. In entry 402 a, values for ports one and two are set toblock the packet from being sent out from those ports. An associatedentry 402 b in trunk block table 400, as shown in FIG. 4 b, associatedwith device 304 is also set. In entry 402 b, values for ports four andsix are set to block the packet from being sent out from those ports. Ondevice 306, an associated entry 402 c, as shown in FIG. 4 c, is set inassociated trunk group table 400. In entry 402 c, the value for port tenis set to block packets from being sent out on that port, however, thevalue for port 11 is set to allow packets and as such the packet istransmitted on port 11.

Although in one embodiment of the invention trunk group table 400 ineach of devices 302-308 is a static 16 entry table, each of the 16entries in each trunk group table can be programmed in different waysfor a specific trunk group. To ensure consistency and that the sameentry is selected in each table 400 across the network, i.e., that thesame entry is selected in the trunk group table associated with device302, in the trunk group table associated with device 304 and in thetrunk group table associated with device 306, in one embodiment of theinvention, if the packet is a layer 2 broadcast, multicast or unknownunicast packet, the sending device uses a predefined number of bits frompredefined fields to determine the trunk selection. Specifically, fourbits are selected from each of the source address, destination addressand the source port trunk group identifier and the selected fields areXORed to obtain a value that is used to index table 400. Alternatively,four bits are selected from each of the source address and thedestination address and two bits are selected from each of source portand source module and the selected fields are XORed to obtain a valuethat is used to index table 400. If the packet is a IP multicast packet,a predefined number of bits are selected from each of the source IPaddress, destination IP address and the source port trunk groupidentifier and the selected fields are XORed to obtain a value that isused to index table 400. Alternatively, a predefined number of bits areselected from each of the source IP address and the destination IPaddress and two bits are selected from each of source port and sourcemodule and the selected fields are XORed to obtain a value that is usedto index table 400. In one embodiment of the invention, the predefinednumber of bits is four. Since the information used in the trunk groupselection is the same for each device, each device 302-308 performs thesame hashing operation in order to select the appropriate entry fromtrunk block table 400.

Specifically, in one embodiment of the invention, if the packet is layer2 broadcast, multicast or unknown unicast packet, SA[3:0], DA[3:0],SRC_PORT[1:0] and SRC_MODID[1:0] are XORed to obtain a four bit valuethat is used to index table 400. Alternatively, SA[3:0], DA[3:0],SRC_PORT_TGID[3:0] are XORed to obtain a four bit value that is used toindex table 400. If the packet is IP multicast packet, SIP[3:0],DIP[3:0], SRC_PORT[1:0] and SRC_MODID[1:0] are XORed to obtain a fourbit value that is used to index table 400. Alternatively, SIP[3:0],DIP[3:0], SRC_PORT_TGID[3:0] are XORed to obtain a four bit value thatis used to index table 400.

The above-discussed configuration of the invention is, in a preferredembodiment, embodied on a semiconductor substrate, such as silicon, withappropriate semiconductor manufacturing techniques and based upon acircuit layout which would, based upon the embodiments discussed above,be apparent to those skilled in the art. A person of skill in the artwith respect to semiconductor design and manufacturing would be able toimplement the various modules, interfaces, and tables, buffers, etc. ofthe present invention onto a single semiconductor substrate, based uponthe architectural description discussed above. It would also be withinthe scope of the invention to implement the disclosed elements of theinvention in discrete electronic components, thereby taking advantage ofthe functional aspects of the invention without maximizing theadvantages through the use of a single semiconductor substrate.

With respect to the present invention, network devices may be any devicethat utilizes network data, and can include switches, routers, bridges,gateways or servers. In addition, while the above discussionspecifically mentions the handling of packets, packets, in the contextof the instant application, can include any sort of datagrams, datapackets and cells, or any type of data exchanged between networkdevices.

The foregoing description has been directed to specific embodiments ofthis invention. It will be apparent, however, that other variations andmodifications may be made to the described embodiments, with theattainment of some or all of their advantages. Therefore, it is theobject of the appended claims to cover all such variations andmodifications as come within the true spirit and scope of the invention.

1. A network device for distribution of non-unicast traffic on a trunkgroup by selecting an appropriate port from the trunk group on which totransmit a non-unicast packet, the network device comprising: at leastone trunk group comprising a plurality of physical ports, wherein thenetwork device is connected to at least one other network device by anumber of the plurality of physical ports; a table comprising aplurality of entries, wherein each entry is associated with the numberof the plurality of physical ports on the network device and each entryspecifies if a packet should be transmitted on one of the plurality ofphysical ports; hashing means for hashing a predefined number of bitsfrom predefined fields in the packet to select one entry in the table,wherein the selected entry is used to identify a destination port; andtransmitting means for transmitting the packet to the destination port.2. The network device according to claim 1, wherein the table isconfigured as a 16 entry table, wherein each entry comprises a fieldthat is associated with one of the number of the plurality of physicalports.
 3. The network device according to claim 1, wherein the table isconfigured to set one field that is associated with one of the pluralityof physical ports to allow the packet transmission.
 4. The networkdevice according to claim 1, wherein the table is configured to setfields that are associated with the number of the plurality of physicalports to block the packet transmission.
 5. The network device accordingto claim 1, wherein the table is configured such that entries in thetable are programmed in different way for the trunk group.
 6. Thenetwork device according to claim 1, wherein the hashing means hashesthe predefined number of bits to obtain an index for accessing one entryin the table.
 7. The network device according to claim 6, wherein thehashing means hashes, if the packet is one of a layer 2 broadcast,multicast or unknown unicast, a predefined number of bits from a sourceaddress, a destination address and a source port trunk group identifierto obtain the index for accessing one entry in the table.
 8. The networkdevice according to claim 6, wherein the hashing means hashes, if thepacket is one of a layer 2 broadcast, multicast or unknown unicast, apredefined number of bits from a source address, a destination address,source port and a source module identifier to obtain the index foraccessing one entry in the table.
 9. The network device according toclaim 6, wherein the hashing means hashes, if the packet is an IPmulticast, a predefined number of bits from a source IP address, adestination IP address and a source port trunk group identifier toobtain the index for accessing one entry in the table.
 10. The networkdevice according to claim 6, wherein the hashing means hashes, if thepacket is an IP multicast, a predefined number of bits from a source IPaddress, a destination IP address, a source port and a source moduleidentifier to obtain the index for accessing one entry in the table. 11.A method for uniform distribution of non-unicast traffic on a trunkgroup by selecting an appropriate port from the trunk group on which totransmit a non-unicast packet, the method comprises the step of:connecting the network device to at least one other network device by anumber of a plurality of physical ports in at least one trunk group;storing a plurality of entries in a table, wherein each entry isassociated with the number of the plurality of physical ports;specifying in each entry if a packet should be transmitted on one of theplurality of physical ports; hashing a predefined number of bits frompredefined fields in the packet to select one entry in the table;identifying a destination port by a selected entry; and transmitting thepacket to the destination port.
 12. The method according to claim 11,further comprising storing at least one field in each entry, wherein theat least one field is associated with one of the number of the pluralityof physical ports.
 13. The method according to claim 11, furthercomprising setting one field that is associated with one of theplurality of physical ports to allow the packet transmission.
 14. Themethod according to claim 11, further comprising setting fields that areassociated with the number of the plurality of physical ports to blockthe packet transmission.
 15. The method according to claim 11, furthercomprising programming entries in the table in different way for thetrunk group.
 16. The method according to claim 11, further comprisinghashing the predefined number of bits to obtain an index for accessingone entry in the table.
 17. The method according to claim 16, furthercomprising hashing, if the packet is one of a layer 2 broadcast,multicast or unknown unicast, a predefined number of bits from a sourceaddress, a destination address and a source port trunk group identifierto obtain the index for accessing one entry in the table.
 18. The methodaccording to claim 16, further comprising hashing, if the packet is oneof a layer 2 broadcast, multicast or unknown unicast, a predefinednumber of bits from a source address, a destination address, source portand a source module identifier to obtain the index for accessing oneentry in the table.
 19. The method according to claim 16, furthercomprising hashing, if the packet is an IP multicast, a predefinednumber of bits from a source IP address, a destination IP address and asource port trunk group identifier to obtain the index for accessing oneentry in the table.
 20. The method according to claim 16, furthercomprising hashing, if the packet is an IP multicast, a predefinednumber of bits from a source IP address, a destination IP address, asource port and a source module identifier to obtain the index foraccessing one entry in the table.
 21. An apparatus for uniformdistribution of non-unicast traffic on a trunk group by selecting anappropriate port from the trunk group on which to transmit a non-unicastpacket, the apparatus comprising: connecting means for connecting thenetwork device to at least one other network device by a number of aplurality of physical ports in at least one trunk group; storing meansfor storing a plurality of entries in a table, wherein each entry isassociated with the number of the plurality of physical ports;specifying means for specifying in each entry if a packet should betransmitted on one of the plurality of physical ports; hashing means forhashing a predefined number of bits from predefined fields in the packetto select one entry in the table; identifying means for identifying adestination port by a selected entry; and transmitting means fortransmitting the packet to the destination port.